1. Field of the Invention
The present invention relates to a suck back valve capable of preventing liquid drip of a fluid from occurring at a supply port, by sucking a prescribed amount of fluid flowing through a fluid passage based on a displacement operation of a diaphragm.
2. Description of the Related Art
There has hitherto been known, for example, a semiconductor wafer manufacturing process in which a suck back valve is employed. In the suck back valve, when supply of a coating liquid to the semiconductor wafer is halted, there is a function for preventing so called liquid drip, wherein minute amounts of coating liquid drip from a supply port onto the semiconductor wafer.
The suck back valve according to such a conventional technique is shown in FIG. 6 and is disclosed, for example, by Japanese Utility Model Publication No. 8-10399.
Such a suck back valve 1 comprises a main valve body 5 formed with a fluid passage 4 interconnecting a fluid inlet port 2 and a fluid outlet port 3, and includes a bonnet 6 connected to an upper portion of the main valve body 5. A diaphragm 7 made up of a thick-walled portion and a thin-walled portion is disposed in the center of the fluid passage 4. An unillustrated pressurized fluid supply source is connected to the bonnet 6, the bonnet further being formed with a pressurized fluid supply port 8 for supplying pressurized air for operating a diaphragm under a switch-over action of a directional control valve (not shown).
A piston 9 is fitted to the diaphragm 7, with a v-packing 10 installed in the piston 9 and slidable along an inner wall surface of the main valve body 5 and serving a sealing function. Further, a spring 11 which normally presses the piston in a upward direction, is disposed in the main valve body 5.
An adjusting screw 12 is disposed on the top of the bonnet 6, which, by increasing or decreasing a screw-in amount thereof, abuts against the piston 9 and adjusts a displacement amount of the piston 9, whereby the amount of coating liquid sucked by the diaphragm 7 is adjusted.
A coating liquid supply source 13 storing therein a coating liquid is connected to the fluid inlet port 2 through a pipe 14. Further, between the coating liquid supply source 13 and the fluid inlet port 2, and ON/OFF valve 15, which is constructed as a separate body apart from suck back valve 1, is connected. The ON/OFF valve undertakes a function for switching between a supply condition and a supply-halted condition of the coating liquid to the suck back valve 1, based on energizing and de-energizing actions of the ON/OFF valve 15.
Operation of the suck back valve 1 shall now be explained in outline. In an ordinary state in which the fluid is supplied from the fluid-inlet port 2 to the fluid-outlet ports, the piston 9 and diaphragm 7 are displaced downwardly in an integrated manner in accordance with action of the pressurized fluid supplied from the pressurized fluid supply port 8. The diaphragm 7, which is coupled to the piston 9, protrudes into the fluid passage 4 as shown in FIG. 5 by the two-dot chain lines.
When flow of fluid through the fluid passage 4 is halted, the piston 9 and diaphragm 7 are raised in unison under the action of an elastic force exerted by the spring 11, by stopping the supply of pressurized fluid from the pressurized fluid supply port 8. A predetermined amount of fluid remaining in the fluid passage 4 is sucked under action of a negative pressure produced by the diaphragm 7. Thus, dripping of liquid, which would otherwise be caused at an unillustrated fluid supply port, is prevented.
In this instance, the suck back amount of the coating liquid corresponds with displacement of the piston 9, wherein a displacement amount of the piston 9 is adjusted by the screw element 12.
Incidentally, using the suck back valve 1 according to the conventional technique, in order to adjust with high precision the flow amount of pressurized air which is supplied to the pressurized fluid supply port 8, a fluid pressure control apparatus 17 is connected to the pressurized fluid supply port 8 through a pipe 16 such as tube or the like. The fluid pressure control apparatus 17 is formed as a separate body apart from the suck back valve 1, and comprises a piezoelectric element bimorph cell arranged therein which converts an electric signal into air pressure.
However, in the suck back valve 1 according to the conventional technique, because adjustment of the coating liquid suck back amount is performed by manually increasing/decreasing the screw-in amount of the screw member 12, there is the inconvenience that precise control of the coating liquid suck back amount cannot be achieved. In this case, it becomes necessary to readjust each time the amount at which the preset screw member 12 is screwed in, corresponding with the amount of supply of the coating liquid, resulting in further complexity.
Further, with the suck back valve according to the conventional technique, because there is some difficulty in reproducing the displacement amount, with respect to a voltage imposed by a hysteresis action possessed by the piezoelectric element bimorph cell disposed in the fluid pressure control apparatus, it becomes difficult to control with precision the flow rate of pressurized air supplied to the pressurized fluid supply port 8, with the further inconvenience that displacement speed of the piston when the coating liquid is sucked cannot be precisely controlled.
Further, when the suck back valve 1 according to the conventional technique Is used, complications arise in that it becomes necessary to undertake pipe connecting operations between the suck back valve 1 and the fluid pressure control apparatus 17, as well as between the suck back valve 1 and the ON/OFF valve 15. In addition, because the fluid pressure control apparatus 17 and ON/OFF valve 15 are respectively installed externally of the suck back valve 1, dedicated space for such elements must be provided, increasing the installation space overall.
Further still, flow passage resistance increases as a result of piping connected between the fluid pressure control apparatus 17 and the suck back valve 1, with the inconvenience that responsiveness and precision of the diaphragm becomes deteriorated.
Even further, a driving device (not shown) for switching between ON and OFF states of the ON/OFF valve 15 must be separately provided, wherein in addition to the complexity of further pipe connecting operations for the ON/OFF valve 15 and the driving apparatus, soaring costs for the apparatus are incurred.